1. Field of the Invention
The present invention relates to optical fibers and more precisely to reducing the polarization mode dispersion of such fibers during fabrication.
2. Description of the Prior Art
In an ideally circular monomode optical fiber there are two polarization modes: if the fiber is circular, the two modes propagate along the fiber with identical phase velocities. However, any asymmetry of the fiber, for example caused by a non-circular core; asymmetrical lateral stresses or index variations, induces a difference between the phase velocities of the two inherent modes, and this is known as birefringence. The birefringence of a fiber is measured by a parameter called the polarization mode dispersion, which is representative of the dispersion induced in a signal by the birefringence of the fiber. This problem of defective cylindricality of optical fibers, as just explained, arises not only with monomode optical fibers but also with other types of optical fibers.
EP-B-0 582 405 describes a standard fiber drawing installation in which a preform is heated at its end; the heated end of the preform is drawn to form an optical fiber. The installation includes diverse stations for controlling the diameter, applying a coating, controlling the concentricity and thickness of the coating, hardening the coating. The fiber is wrapped around pulleys and driven by a drawing capstan before it is wound onto a spool or drum. To reduce polarization mode dispersion, the above document proposes that one of the fiber guide pulleys be subjected to an alternating rotation movement about an axis in the fiber drawing plane. The effect of this oscillatory movement is to cause the fiber to roll on the surface of the pulley, on either side of a fiber drawing plane defined by the axis of the preform and the fiber. This lateral movement of the fiber induces torsion in and permanent deformation of the fiber being drawn. The effect of the alternating movement of the pulley is to apply torsion to the fiber in alternating directions, which reduces polarization mode dispersion. EP-A-0 744 636 proposes a solution that avoids the alternating movement of the pulley by using a simple rotation movement of a pulley having a skewed surface. As it rotates, the skewed pulley offers up to the fiber a surface equivalent to that of a wheel of circular symmetry whose axis is subjected to an alternating movement.
WO-A-98 46536 describes another system for applying torsion to a fiber in which members are in contact with the fiber, on respective opposite sides thereof. One of the members is subjected to alternating movement.
One problem with this solution is that of determining the torsion applied to the fiber or the rotation of the fiber on the oscillating device. This problem is especially complex because the fiber has a dimension that is typically of the order of 250 μm, a cylindrical shape and is drawn at velocities that can be as high as 1 500 m/min.
WO-A-01 33184 indicates that it is known in the art to introduce bubbles or other irregularities into the fiber. It is then possible, for a given adjustment of the oscillatory movement of the pulley, to measure the torsion applied to the drawn fiber. Nevertheless, a method of this kind implies drawing the fiber before being able to carry out the measurements and leads to a waste of time and fiber. It further implies a new measurement each time the characteristics of the drawing device (preform, type of coating, fiber drawing velocity, etc.) are changed. Finally, the measurement is not carried out on the fiber itself, but on a specific preform, which gives rise to a problem with the reproducibility of the method.
WO-A-01 33184 also proposes to measure the diameter of the fiber, and then to calculate a Fourier transform of the diameter. The corresponding power spectrum has peaks at a frequency of 2v where v is the frequency at which the oscillation of the pulley is reversed. The amplitude of the torsion is obtained from the spectrum, by calculating the amplitudes of the peaks of the spectrum. This solution presupposes the availability of apparatus for precisely measuring the diameter of the fiber and sophisticated computation means. This solution is also limited by its very nature to periodic oscillations of the system for preventing polarization mode dispersion.
There is therefore a need for a solution for controlling the polarization mode dispersion of an optical fiber when it is being drawn from a preform. The solution must be simple to put into practice, reliable, and not necessitate a specific preform. It must also be applicable in real time, or virtually in real time, without implying complex measurements or complex computations, and for all types of oscillation of the system for preventing polarization mode dispersion.